Reference is made to commonly assigned published European patent EP-A-O 265,918 and to the corresponding copending U.S. Pat. No. 4,955,014 filed on 30 Oct. 1987 in the name of D. Kuppers et al. In the referenced patent documents, an embodiment of a diplex transmission system, i.e. a multiplex system for transmission of two signals unidirectionally via a single waveguide, is illustrated in FIG. 5. In that embodiment, both signals are transmitted with the same wavelength, but with different waveguide modes, so that the transmission process can be described as a waveguide mode multiplex process. The two optical signals to be transmitted in the different modes are coupled in the waveguide, using a monomode fiber-fusion coupler which is fabricated from two exactly similar monomode fibers, i.e., the multiplexer is a waveguide fusion coupler.
In the following, the simpler expression "fiber" is used instead of the expression "waveguide" from time to time.
With the two exemplary embodiments of FIG. 3 and FIG. 4, respectively, the referenced patent document discloses an optical duplex communications system, i.e., a bidirectional optical communications system for two optical signals. The embodiment according to FIG. 3 deals with a wavelength multiplex system in which both signals are transmitted in the same waveguide mode, i.e., the fundamental mode; and the embodiment according to FIG. 4 deals with a waveguide mode multiplex system in which both signals have the same wavelength. Wavelength-selective waveguide fusion couplers are provided as duplexers to separate the two wavelengths in the embodiment according to FIG. 3. Mode filters (MF2, MF1) are provided as duplexers to separate the two different modes in the embodiment according to FIG. 4; and also in the diplex transmission system according to FIG. 5, a suitable mode filter (MF) is present on the reception side to separate the two modes. Concerning these mode filters, it is stated that in the simplest case they are "tapered" standard monomode waveguides as in FIG. 6 or that they may also be couplers such as are disclosed in another literature citation. However, the couplers disclosed in that citation are not "fusion" couplers, but rather "mechanically polished" couplers.
The principle that the transmission wavelengths are clearly below the cutoff frequency of the waveguide is common to all the examples mentioned. The multiplexers, demultiplexers, or duplexers are in all cases designed to separate the optical signals from each other on the basis of a single characteristic, either on the basis of their wavelength or on the basis of the waveguide mode in which they are propagate via the waveguide, or, in the case of the multiplexer, to combine them into a multiplexed signal.
In each case, the objective is to achieve the most efficient coupling or decoupling when combining or separating the two optical signals. It can be difficult to produce such couplers with the required efficiency, for example, when the two wavelengths are close together when wavelength multiplexing is used.